Shaped like a snow cone, A19 bulbs are what you likely think of when someone says "light bulb". The “A” stands for arbitrary, and in the case of bulbs we take the definition of that to mean the shape was decided upon based on random choice of its designer. The number that follows the “A” in these bulbs indicates the bulb’s size when measured in 1/8” increments. For instance, a standard A19 bulb diameter measures at 19/8”, or when converted is 2.375”.
The typical shape of this general service tool is the universal symbol for light bulbs the world, over! Ranging in outputs from 20 Watts to 100 Watts and beyond, the A19 is the standard for household lighting.